The Moloney sarcoma virus genome consists of two parts, a sarcoma specific part (2,000 nucleotides) and a common part (5,500 nucleotides). The Moloney leukemia virus has the common sequences and a leukemia specific region of 4,500 nucleotides. The different regions have different homologies to known type C viral genomes. The sarcoma-specific sequences are unique except for HT-1 sarcoma virus which has a complete copy of the Moloney sarcoma-specific RNA. The common region is shared in part with all murine sarcoma viruses and SSAV as well. The leukemia specific region is present in none of the murine sarcoma viruses but is homologous in part with regions of all leukemia viruses. The sarcoma specific region is present in all mouse DNA's in one closely matched copy. There are related sequences in some rodents - hamsters, Praomys, rats. These sarcoma specific sequences are expressed in M-MSV transformed cell RNA but not in RNA of cells transformed by chemicals or other agents.